As one of silicon optical waveguides, there is a silicon rib type optical waveguide of non-patent document 1 (“High extinction ratio optical switching independently of temperature with silicon photonic 1×8 switch”, Nakamura et al., OFC2012, OTu2I.3), which makes the confinement in the lateral direction strong by providing a projection shape of silicon of approximately 1 μm width to a waveguide of a silicon thin film called a slab formed onto a substrate. This rib type optical waveguide can suppress formation of higher-order modes in the waveguide, and can suppress PDL (Polarization Dependent Loss).
The size of the principal mode of light of this rib type optical waveguide is about 1 μm, while the size of the principal mode of light of a usual single mode fiber is large and is about 9 μm. When a single mode fiber and a rib type optical waveguide are connected together, a coupling loss is large because of such difference between the sizes of principal modes of light.
As one of methods to reduce such coupling loss, there is introduction of a spot-size converter of a core expansion type that makes the diameter of the principal mode of light of a rib type optical waveguide large by gradually enlarging a core toward an optical fiber. A plurality of spot-size converters have been proposed up to now, and, for example, there has been proposed, as disclosed in patent documents 1-5 (Japanese Patent Application Laid-Open No. 2001-033642, Japanese Patent Publication No. 4719259, Published Japanese translation of PCT application No. 2001-510589, U.S. Pat. No. 7,088,890 and International Publication No. WO 2012/04279), an optical waveguide having a spot size conversion function made by stacking two or more silicon core layers whose widths widen gradually.
Methods to create an optical waveguide having such multi-stage spot size conversion function include a method to form an optical waveguide having a spot size conversion function by: making an additional core layer regrow thick over a wide area including an area for forming the spot size conversion function; and applying etching after that.
However, as shown in FIG. 18, in this manufacturing method, a barrier layer 903 which extends in the direction crossing a rib type optical waveguide at right angles occurs between the region of the optical waveguide in the side of an optical function device and the region of the spot size conversion function in which the core layer have been made grow. The reason of this is that the region of the optical waveguide in the device side and the region of the spot size conversion function are formed separately. Specifically, it is caused by forming a mask with a margin in the boundary area between the two above-mentioned regions formed separately because one of the regions is formed after the other region is covered by a mask. When the barrier layer 903 exists, it is concerned that coupling to higher-order modes and transmission loss by reflection is caused. Regarding such transmission loss, it is known that the transmission loss can be suppressed by introducing taper structures 901-1 and 901-2 disclosed in non-patent document 2 (“Taper-Integrated Multimode-Interference Based Waveguide Crossing Design”, Chyong-Hua Chen, IEEE Journal of Quantum Electronics, VOL. 46, NO. 11 and pp 1656-1661) into optical waveguides before and behind the barrier layer as shown in FIG. 18.